EGR (Exhaust Gas Recirculation) systems are used by major diesel engine manufacturers to reduce emissions of undesirable nitrogen oxides (NOx) from diesel engines. Depending on engine operating conditions, these systems can divert from about 5 to 50 percent of an engine's exhaust stream through a cooler, and then back into the combustion chambers, where the cooled gases reduce peak temperatures, and thus retard NOx formation.